1. Field of the Invention
This invention relates to a method and apparatus for drilling a secondary borehole from an existing borehole in geologic formations and more particularly, to a tapered window mill and whipstock combination that in one trip, can drill a deviated borehole from an existing earth borehole or complete a side tracking window in a cased borehole.
2. Background
Traditionally, whipstocks have been used to drill a deviated borehole from an existing earth borehole. The whipstock has a ramp surface which is set in a predetermined position to guide the drill bit on the drill string in a deviated manner to drill into the side of the earth borehole. In operation, the whipstock is set on the bottom of the existing earth borehole, the set position of the whipstock is surveyed, the whipstock is properly oriented for directing the drill string in the proper direction, and the drilling string is lowered into the well into engagement with the whipstock causing the whipstock to orient the drill string to drill a deviated borehole into the wall of the existing earth borehole.
Previously drilled and cased wellbores, for one reason or another, may become non-productive. When a wellbore becomes unusable, a new borehole may be drilled in the vicinity of the existing cased borehole or alternatively, a new borehole may be sidetracked from or near the bottom of a serviceable portion of the cased borehole. Sidetracking from a cased borehole is also useful for developing multiple production zones.
Sidetracking is often preferred because drilling, casing and cementing the borehole is avoided. This drilling procedure is generally accomplished by either milling out an entire section of casing followed by drilling through the side of the now exposed borehole, or by milling through the side of the casing with a mill that is guided by a wedge or "whipstock" component.
Drilling a side tracked hole through casing made of steel is difficult and often results in unsuccessful penetration of the casing and destruction of the whipstock. In addition, if the window is improperly cut, a severely deviated dog leg may result rendering the sidetracking operation unusable.
Several patents relate to methods and apparatus to sidetrack through a cased borehole. U.S. Pat. No. 4,266,621 describes a diamond milling cutter for elongating a laterally directed opening window in a well casing that is set in a borehole in an earthen formation. The mill has one or more eccentric lobes that engage the angled surface of a whipstock and cause the mill to revolve on a gyrating or non-fixed axis and effect oscillation of the cutter center laterally of the edge thus enhancing the pipe cutting action.
The foregoing system normally requires at least three trips into the well in the sidetracking operation. A first stage begins a window in the casing, a second stage extends the window through use of a diamond milling cutter and a third stage with multiple mills elongates and extends the window. While the window mill is aggressive in opening a window in the casing, the number of trips, such as three, to accomplish the task is expensive and time consuming.
Typically window mills are designed with a square bottom, i.e. a square cross-section. As is shown in FIG. 14, a prior art square bottomed, cross-sectioned mill provides a point of contact between the mill and the whipstock and a large axial surface contact between the mill and the casing. As can be appreciated from FIG. 14, the contact area between the square bottomed mill and whipstock is substantially a line contact while the contact area between the mill and casing is much greater. The applied force, due to the weight on bit, per contact area determines the contact stress between the members. Because the contact stress between the mill and the casing is much greater than the contact stress between the mill and whipstock, the mill tends to cut into the whipstock rather than into the casing even where the cutability of the whipstock has been reduced because of hardfacing.
U.S. Pat. Nos. 2,216,963; 3,908,759; and 4,397,355 disclose mills having a taper or tapered nose. A starter mill with a tapered nose will eventually wedge and cannot complete the window or drill the lateral borehole. U.S. Pat. No. 3,908,759 appears to disclose a taper on the mill. U.S. Pat. No. 2,216,963 discloses a tapered mill which is used in a second trip into the well to increase the window after a square bottomed mill opened the window in a previous trip into the borehole. These patents do not teach guiding and moving these tapered mills laterally through the casing so that at least the center of the downwardly facing cutting surface of these mills passes outside the exterior wall of the casing in one trip into the borehole. At least two trips are required into the well, typically using a starter mill in the first trip to begin cutting a window in the casing and then a second mill in a second trip to increase the window. Further, tapered mills are typically less than full gauge requiring additional trips into the borehole to complete the window.
Weatherford Enterra offers a mill which has a taper extending upwardly and inwardly from a full diameter cutting base. The mill also includes a support shoulder on the cutting face of the mill. However, the reduced diameter taper extends above the full diameter cutting gage of the mill which therefore tends to cut the whipstock rather than the casing.
U.S. Pat. No. 5,109,924 teaches a one trip window cutting operation to sidetrack a wellbore. A deflection wedge guide is positioned behind the pilot mill cutter and spaced from the end of a whipstock component. The shaft of the mill cutter is retained against the deflection wedge guide such that the milling tool frontal cutting surface does not come into contact with the ramped face of the whipstock. In theory, the deflection wedge guide surface takes over the guidance of the window cutting tool without the angled ramp surface of the whipstock being destroyed.
However, when a second and third milling tool attached to the same shaft as the window milling cutter and spaced, one from the other on the support shaft contacts the whipstock ramped surface, they mill away the deflection guide projection from the ramp surface. This inhibits or interferes with the leading pilot mill window cutter from sidetracking at a proper angle with respect to an axis of the cased borehole and may cause the pilot window cutting mill to contact the ramp surface of the whipstock before the pilot window cutter mill clears the casing. The reamers or mills aligned behind the pilot window mill, having the same or larger diameter than the diameter of the pilot window mill, prevents or at least inhibits the window pilot mill from easily exiting from the steel casing. This difficulty is due to the lack of clearance space and flexibility of the drill pipe assembly making up the one trip window cutting tool when each of the commonly supported reamer mills spaced along the shaft, sequentially contact the window in the steel casing. Hence, the sidetracking apparatus tends to go straight rather than be properly angled through the steel pipe casing.
U.S. Pat. No. 5,445,222 teaches a combination whipstock and staged sidetrack mill. A tapered, cone-shaped mill is located on the end of a common shaft and has an outer diameter of about 50 to 75 percent of the maximum diameter to which the final sidetracked hole will be completed. Three stages of cutting mills are disposed above the tapered mill on the common shaft. Each successive stage increases in diameter. A surface of a second stage cutter is, at its smallest diameter, about the diameter of the maximum diameter of the tapered mill, and is, at its largest diameter, at least 5 percent greater in diameter than the diameter of the tapered mill. A surface of a final stage cutter mill is, at its largest diameter, about the final diameter dimension, and at the smallest cutting surface diameter, is a diameter of at least about 5 percent smaller than the final diameter dimension. The whipstock guide is made of a material that is harder than the casing but not as hard as the cutting elements of the mill whereby the mill is to cut the casing rather than the whipstock.
The sidetracking mill is designed to accomplish the milling operation in one trip. The mill however, tends to go straight and penetrate the ramped surface of the whipstock. Substantial damage to the whipstock occurs and sidetracking may not occur as a result.
While the intent is to perform a sidetracking operation in one trip, difficulties often arise when attempting to deviate the drill string from its original path to an off line sidetracking path. Progressively larger in diameter reaming stages to enlarge the window in the steel casing inhibits the drill shaft from deviating or flexing sufficiently to direct the drill pipe in a proper direction resulting in damage to the whipstock and misdirected sidetracked boreholes. In other words, the sidetracking assembly tends to go straight rather than deviating through the steel casing.
The present invention overcomes these deficiencies in the prior art.